Traditionally, LCD displays had a fixed refresh rate, wherein the contents of the screen are refreshed at fixed time intervals, e.g., at 60 Hz. While fixed refresh rates performed adequately for certain applications, e.g., T.V. shows, other applications, e.g., gaming suffered. Depending on the complexity of the calculations, the graphics processing unit (GPU) used to render gaming graphics for an LCD display renders frames at varying rates. The difference in the rendering rate of the GPU and the fixed refresh rate of the LCD results in conspicuous visual artifacts that distort a user's experience of the game.
Variable refresh rate monitors alleviate this problem by requiring the LCD screen to sync with the GPU instead of refreshing at a fixed rate. The GPU sends an image to the LCD as soon as it is rendered and the LCD monitor repaints the image. Subsequently, the LCD waits for the next image to be transmitted from the GPU. This reduces artifacts like stutter and tearing and results in smoother on-screen motion. However, because of the variable refresh rate, each RGB component of a pixel can start to accumulate charge if positive and negative polarity frame durations are not equal because of an unbalanced polarity pattern (also called a beat pattern).
The intensity of each one of the RGB components of a pixel of a liquid crystal display (“LCD”) is determined by the voltage difference that is applied to the pixel cell. In the neutral state, no voltage is applied. In the active state, the voltage can either have positive or negative polarity. It should be noted that both positive and negative polarities result in the same intensity of color on the LCD screen. As the voltage is applied to a pixel cell, the RGB component of a pixel (hereinafter, each RGB component of a pixel will be referred to as a “pixel”) may slowly accumulate a charge. When this charge is present, the intensity of the pixel will be different than when the charge is not present, even in cases where the same voltage is applied.
Over time, the charge accumulation inside a component dot of a pixel will result in visual artifacts. For example, the intensity of the pixel will be different when a positive voltage is applied than when a negative voltage is applied. If the polarity changes for each frame displayed, the pixel will alternately have different values, which can be observed as flicker.
To avoid this charge accumulation and flicker, the driving electronics of the LCD panel need to ensure that the average charge in the pixels stays close to zero, which means that the average voltage applied over time should approximately be zero also. It should be noted that because the charge inside a pixel leaks away over time, similar to a leaky capacitor, the average voltage applied does not have to be exactly zero.
In a display with a fixed refresh rate, ensuring that the average voltage applied is zero can be accomplished by alternately applying a positive and negative voltage across the pixels. The polarity of the voltage on each pixel is typically changed for each frame for a regular 2D display, e.g., in the following pattern:(+−+−+−+−). For some stereo 3D displays, for example, the polarity of the voltage on each pixel may change in the following fashion: (++−−++−−++−−++).
In a variable rate display, however, ensuring that the average voltage charge applied stays close to zero is more challenging. Conventional variable rate LCD displays do not have an efficient mechanism for ensuring that the average voltage applied over time stays close to zero and, therefore, undesirable parasitic charge can build up for the pixels of the LCD screen which causes visible artifacts.